[LAU] Issue with the priority of the sound cards using rtirq

Fernando Lopez-Lezcano nando at ccrma.Stanford.EDU
Thu May 3 06:25:56 UTC 2012

On 05/02/2012 12:19 AM, Jeremy Jongepier wrote:
> On 05/01/12 20:32, Fernando Lopez-Lezcano wrote:
>> (I don't seem to find the start of the thread in my mailbox)
>> ohci_hcd is there is you specify usb in the list of priorities rtirq
>> needs to raise. And nvidia is there probably because it shares an
>> interrupt with a real soundcard. Rtirq does not raise the priority of
>> only the soundcard, but everything that uses the interrupt that the
>> soundcard uses.
> Afaik this is not true. The threadirqs kernel option (or a real-time
> kernel) creates bottom halves (or tasklets, or softirqs) of each device
> that has or shares an IRQ. So in Ralf's case four devices share IRQ 18,
> two USB hubs, a soundcard and a graphics card. Each of those devices
> gets its own specific softirq which then can get prioritized with rtirq
> accordingly. Default rtprio is 50 afaik, so nvidia should get rtprio 50
> and not 82.

That's how we all would like things to work, regretfully that is not how 
things actually work.

>> AFAIK it is not possible (or simple) to raise _just_ the soundcard as it
>> is impossible to know which irq process corresponds to a soundcard (and
>> which one does not). That is because the labeling of the irq process
>> names in the ps listing is arbitrary.
> It is not arbitrary, the softirqs get specific names derived from the
> loaded kernel module for that device iirc.

For soundcard drivers there is no convention for naming the irq process 
that services its interrupt. The name of the irq process does not match 
the name of the kernel module (which would be an obvious choice). The 
ascii that is printed by ps is defined in the kernel code for each 
driver, and driver authors pick different names that are not consistent. 
That means you can't pattern match for the right irq process if the 
interrupt number is shared by several devices.

It would be possible to pattern match if, for example, all soundcards 
used a "snd_" prefix for the name of the irq process, but that is not 
the case (at least until last time I looked at the kernel code for this 
very same reason).

>So the graphics card gets a
> label 18-nvidia, the soundcard 18-snd_hdsp and both usb hubs
> 18-ohci_usb. And it is those labels that rtirq uses to set the rtprio
> for the different softirqs.

The rtirq code does the following (simplified):

- rtirq gets the irq number from /proc/asound/cards
- that number is used to search the output of ps looking for matches 
with that particular irq number
- there is no way to tell which irq process corresponds to a soundcard, 
rtirq does not try AFAICT
- so, all irq processes that match the number get the elevated priority

For example, in my laptop, from /proc/interrupts:

  16:       2551        516        113         95   IO-APIC-fasteoi 
nouveau, snd_hda_intel, mmc0

rtirq happily gives them 75, 74 and 73:

   222 FF      75   - 115  0.0 S    irq/16-nouveau
11077 FF      74   - 114  0.0 S    irq/16-snd_hda_
11082 FF      73   - 113  0.0 S    irq/16-mmc0

Please let me know if there is a way to, given an irq number, know what 
is the pid of the irq process that services that irq number for a 
particular _soundcard_, I have found no way of doing that so far[*].

Note that in all other cases other than soundcards rtirq looks for the 
name, not the number of the irq (for example for usb the names are well 
known and unique and hardwired into rtirq).

-- Fernando

[*] other than scanning the kernel source code for the names of all 
possible soundcard driver irq processes, making a list and matching 
against that - it would work but it would depend on those names not 
changing, and of having the list that matches the kernel you are running.

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